This invention concerns a continuous process for the manufacture of dianhydro sugar alcohols by the dehydration of the corresponding sugar alcohols in the presence of a dehydration catalyst, using evolved water vapor as a carrier for continuous separation of the product from the reaction mass.
Anhydro sugar alcohols, in particular derivatives of mannitol, iditol, and sorbitol, are known for their therapeutic uses and uses in food. At least one of these, isosorbide, 1,4:3,6-dianhydrosorbitol, is useful as a monomer used in the manufacture of polymers and copolymers, especially polyester polymers and copolymers.
Anhydro sugar alcohols are produced by dehydration of the corresponding sugar alcohols (or monoanhydro sugar alcohols) by the action of various dehydration catalysts, typically strong acid catalysts such as sulfonated polystyrenes (H+ form) and various mineral acids (e.g., HCl, H3PO4, HF, H2SO4). Batch processes for the preparation of dianhydro sugar alcohols by acid dehydration are known in the art.
It is known that the dehydration of sorbitol to isosorbide proceeds at a very slow rate if water is not removed effectively from the reaction mass. G. Flxc3xa8che and M. Huchette, Starch/Starke (1986), 38(c), 26-30, have shown that at 135xc2x0 C., the reaction may take as long as 10 to 15 hours to complete if carried out at atmospheric pressure, but with removal of water under vacuum, the reaction time is reduced to 2 to 3 hours. Organic solvents such as xylene and toluene have also been employed to facilitate the removal of water (Przem. Chem. 48(11) pp. 665-668 (1969)).
A batch process for the formation of the dianhydro sugar alcohol isosorbide has been described as a two step process involving intramolecular dehydration of sorbitol to sorbitan (1,4-monoanhydrosorbitol), and further reaction of sorbitan to isosorbide (1,4:3,6-dianhydrosorbitol) in an acid catalyzed dehydration-cyclization. In this process, an aqueous solution of sorbitol is charged to a batch reactor. The temperature is increased to 130 6C.-135xc2x0 C. under vacuum (35 mm Hg) to remove the water. When most of the free water has been removed, a catalyst, usually sulfuric acid, is added and the temperature and vacuum levels are maintained. The operable temperature range of the reaction is narrow. Higher temperatures lead to decomposition and charring of the end product, while lower temperatures reduce the reaction rate and make it difficult to remove the water of reaction. This reaction produces isosorbide and a higher molecular weight by-product. The by-product is presumably produced by water elimination between two or more sorbitol molecules, but its exact nature is not clearly defined. See G. Flxc3xa8che and M. Huchette, Starch/Starke (1986), 38(c), 26xe2x89xa730 and Roland Beck, Pharm. Mfg Inc. (1996), 97-100. Other monoanhydro by-products, 2,5-anhydro-L-iditol and 2,5-anhydro-D-mannitol, are also known to be produced under some reaction conditions (K. Bock et al., Acta. Chem. Scand. B 35, 441-449 (1981)).
For isosorbide to be used as a monomer in high volume polymers and copolymers, it needs to be produced in large quantities, preferably in a continuous process.
WO 00/14081 describes a continuous process for producing anhydro sugar alcohols, especially isosorbide, comprising the steps of introducing at least one sugar alcohol or monoanhydro sugar alcohol into a reaction vessel; dehydrating the sugar alcohol or monoanhydro sugar alcohol in the presence of an acid catalyst and an organic solvent to form a reaction product which is at least partly soluble in the organic solvent; removing water from the reaction vessel; removing organic solvent comprising the dissolved reaction product from the reaction vessel; separating the reaction product from the removed organic solvent; and recycling the organic solvent into the reaction vessel.
U.S. Pat. No. 6,407,266 describes a continuous process in which a process stream containing at least one sugar alcohol or monoanhydro sugar alcohol and, optionally, water is introduced to the first stage of a multistage reactor and then intimately contacted with a countercurrent flow of an inert gas at elevated temperature. This inert gas removes the bulk of any water present in the process stream. This dewatered process stream is then intimately contacted, in the presence of a dehydration catalyst, with a counter current flow of an inert gas at elevated temperatures to remove water of reaction as formed. Finally, the product is removed from the bottom of the reactor.
The reaction product obtained by processes such as the above contains about 70 to 80% isosorbide and about 20 to 30% undesired reaction byproducts. The reaction product thus needs to be subjected to one or more separation steps, such as evaporation, distillation or chromatographic separation, to isolate the isosorbide. Chromatographic separation is disclosed in U.S. patent application Ser. No. 60/246038. Separation by vaporization or distillation is difficult because of the low vapor pressure of isosorbide. At 140xc2x0 C., the vapor pressure of isosorbide is only 1.75 mm Hg. Evaporation or distillation at temperatures of about 140xc2x0 C. is desirable to minimize product degradation and obtain good purity isosorbide, but the recovery is poor. At higher temperatures, e.g., 170xc2x0 C., more isosorbide is recovered, but it is of poorer quality.
The sorbitol used in such processes is typically manufactured as an aqueous solution containing about 50% by weight of sorbitol. This xe2x80x9crawxe2x80x9d solution is then concentrated by evaporation by the manufacturer, typically to about 70% by weight sorbitol. It is this concentrated solution that is used in the above-described processes for making isosorbide. A process that could effectively use the original, more dilute solution would thus be more economical, since the evaporative step would not be necessary.
It is the object of the present invention to provide an economically attractive, continuous process wherein the feed sugar alcohols may be a dilute aqueous solution, the reaction and separation occur simultaneously in a single multistage reactor, and the free water plus the water of reaction evolved are used to facilitate separation.
In accordance with the object of this invention, there is provided a continuous process for the manufacture of dianhydro sugar alcohols comprising:
a) continuously feeding an aqueous solution comprising one or more sugar alcohols or monoanhydro sugar alcohols to a first stage of a multistage reaction vessel;
b) continuously heating the aqueous solution in the first stage in the presence of a dehydration catalyst to form a partially dehydrated reaction mass and water vapor;
c) simultaneously transporting the partially dehydrated reaction mass from the first stage upwards to one or more additional stages of the multistage reactor with water vapor from step (b) while further dehydrating the sugar alcohols to form dianhydro sugar alcohols and simultaneously removing from the reaction mass most of the dianhydro sugar alcohols formed;
d) continuously removing a vapor stream comprising water and dianhydro sugar alcohol product from the reaction vessel and recovering the dianhydro sugar alcohols; and
e) continuously removing high boiling by-products from the reaction vessel as a liquid stream from the final stage of the reaction vessel at a rate coordinated with the feed rate of the aqueous solution so as to maintain a nearly constant quantity of reaction mass in the multistage reaction vessel.